A blood analyzer, which includes an optical flow cytometer, for analyzing a blood sample is known. The flow cytometer is provided with a flow cell for conducting the liquid of the blood sample, a light source for irradiating light onto the flow cell, and a light receiving element, and the light irradiated from the light source is scattered by the particles in the flow cell. The hemolyzing process of the red blood cells and the staining process of the particles of the white blood cells and the like are performed by adding hemolytic agent and fluorescence reagent to the blood sample, and the stained particles emit fluorescence light when receiving light. The scattered light and the fluorescence light are received by the light receiving elements, and the detection signals thereof are analyzed to measure the white blood cells in the blood sample and to classify the white blood cells to lymphocytes, monocytes, granulocytes and the like. In such flow cytometer, since the classification of the white blood cells and the ghosts (red blood cell membrane not completely shrunk with hemolytic agent) is discriminated using the fluorescent light signal, the hemolytic agent having high hemolyzing ability of shrinking the red blood cells to an extent the sizes of the white blood cells and the ghost can be clearly distinguished does not need to be used and thus the extent of damage of the white blood cells is alleviated and the form of the white blood cells is maintained. The reagent for dissolving the red blood cells while maintaining the form of the white blood cells to have the blood sample in a state suitable for the classification of the white blood cells has been disclosed (see U.S. Pat. No. 6,004,816).
Furthermore, an optical system of high measurement accuracy becomes necessary since the classification of the white blood cells by means of the flow cytometer is performed based on the slight difference in size and form of the cell or the nucleus of each white blood cell. Moreover, the optical system of high measurement accuracy is necessary to discriminate the white blood cells and the ghosts using the fluorescent light signal since some ghosts are attached with a small amount of fluorescent pigment and thus emit fluorescent light signal. A photo-multiplier (photoelectron multiplier) having high sensitivity is generally used as the fluorescent light receiving element (see e.g., U.S. Pat. No. 6,365,106). Furthermore, a flow cytometer using an avalanche photodiode (APD) as the light receiving element for receiving fluorescent light is also disclosed (see U.S. Pat. No. 5,739,902).
The amplification ratio of the signal of the element itself is low in the avalanche photodiode compared to the photo-multiplier, and thus the gain of the amplifying circuit arranged in the post-stage of the element must be set large. However, if the output signal of the avalanche photodiode is amplified with the amplifying circuit set with a large gain, the level of high frequency noise generated in the amplifying circuit increases, and high precision analysis of the sample becomes difficult.